Method of and apparatus for producing paper pulp



R. B. WOLF Dec. 22, 1931.

METHOD AND APPARATUS FR PRODUCING PAPER PULP'.

Filed June 5, 1930 INVENTOR if".

V 84m-4r ATTORNEYS Patented Dec. 2.2, 1931 UNI-TED STATES PATENT OFFICEROBERT R. WOLF,

OF NEW YoRR, N.'Y..Ass1eNoR 'ro LIGNO-CEL'LLOSE CORPORATION, OE NEWYORK, N. Y., A CORPORATION oF DELAWARE METHOD 0F AND APPARATUS FORPRODUCING PAPER PULP Application filed .Tune 5,

This invention relates to the production of paper pulp and particularlyto improvements in the sulphite process and in the recovery of the acidemployed therein. i

To secure a proper Yunderstandino` of the invention, it is necessary toreview briefly the development of the sulphite process for treatingpaper pulp which was described originally in the patent to Tilghman No.70,485, November 5, 1867. In this process as originally applied, it wasimpossible to recover the blow-off or relief gases. These gases,consisting principally of sulphur dioxide, were discharged to theatmosphere and lost. The first attempt to recover the gases involved theabsorption thereof in the cooking liquor as described in the DrewsenPatent No. 492,196 dated February 21, 189.3. The latter method wassubject to certain disadvantages, however, and particularly theimpossibility of retaining the sulphur dioxide gas in the acid at thehigh temperature resulting from the introduction of the hot gas thereto.

'The next improvement involved the introduction of so-called reliefcoolers between the digesters and the recovery tanks so that thetemperature of the acid could be kept rela-` tively low, usually between70 and 90 F. With such improvementsit was possible to recover the acidat a temperature of approximately 75 F. at atmospheric pressure,containing approximately 7% of sulphur dioxide, although in the ordinarymill the total sulphur dioxide content of the acid averages between 4and 5%. v Y

The advantage of using strong acid was pointed out in the Wolf PatentNo. 1,699,556, June 22, 1929. That patent also called attention to thefact that strong acid has not been used because the excess ofsulphurdioxide gas remaining in the digester Yat the end of the cook ordinarilycould not be forced over into theA recovery tanks without advancing thel temperatures in the lcooking operation and thereby losing most of thebenefit resulting from the use of strong acid. A method of recoveringstrong acid is described in the Wolf Patent No. 1,097,781, May 26, 1914,and

a simplified and more effective method is de` 1930. Serial No. 459,365.

crlibed in the Wolf Patent No. 1,499,898, July To get the best results,strong acid should be used at low temperature. This principle isrecognized in the patent to Moore and Wolf, 1,110,454, September 15,1914, which relates to a method of blowing the digesters into a blow pitand collecting the vomit stack gases and passing'them through a surfacecondioxide gases in the presence of moisture and because of the largerelief lines necessitated by the volume of gases.

Another step qin the industry was in the direction' of making itpossiblel to get the effect of stronger free acid by another method,namely, the method of 'relieving the digesters into an accumulatorwithout cooling the relief gases in liquids. The first description ofthis method appears in the Clemm Patent No. 1,429,128, September 12,1922, which is followed by the hot acid recovery system of Richterdescribed in Patent No. 1,653,416,

patents. l

' In the hot acid recovery system, the emphasis is upon the recovery ofheat from the relief gases. These gases are conducted, therefore,without cooling to a hot acid accumulator. In the ordinary mill alnumber ofdigesters are connected to a common relief lme wh1ch carriesthe. gases from the digesters to the acld accumulators. This introducesseriousjdifculties, including a wide varia-A tion in the acid strengthand in the temperature of the acid which isintroduced from theaccumulators to thedigesters, but more particularly in the constantvariation of the pres'- December 20, 1927, and in ther and later4 'surein the relief line which seriously affects ,of the respective digesters.It is a matter o f common knowledge in the opera-tio of d1- gesters thatthe pressure therein should remain substantially constant. Th operatorusually sets the relief valve a tie desired opening to maintain, so faras i-s possible, a constant pressure, but this control becomes whollyineffective under the system described, in which the hot relief gasesare conducted through a common relief line to the hot acid accumulators.

`To appreciate this difficulty more fully, it is desirable to considerthe normal operation of a digester. The digester is first filled withchips and with acid, the acid being as nearly as possible of uniformtemperature and uniform chemical composition.` yThe cover of thedigester is secured in place and steam is admitted into the bottom ofthe digester. The pressure is brought up to approXimately 7 5 pounds persquare inch at the top of the digester` in a period ranging from 2 to 3hours. After the pressure hasreached 75 pounds, which is the usualworking pressure, no more steam can be admitted unless gas is relievedfrom the digester. This is accomplished by mea-ns of a relief valve onthe cover. The speed at which the'temperature in the digester is raiseddepends upon the amount of opening of the relief valve. A

wide opening, permitting the` escape of a large amount of gas, lets in alarge amount of steam, and therefore causes a rapid advance intemperature. The reverse is true if the relief valve is only slightly,opened; namely, the temperature will advance slowly. It is possible tocontrol the speed of cooking yso that the variation vin practice isbetween hours and 20 hours, the purpose of the longer cooking time beingto complete the disintegration of the wood chips into fibres at as lowtemperatures as is possible. If the pressure in the relief line variesconstantly, the control of the cooking time is lost substantially, andthe temperature of 'the digester will fluctuate widely because of thevariations in the amount of steam introduced thereto.

` The rate of acceleration of temperature in the cooking operation isvery important. In

\ other words. it makes quite a difference in the character of the pulpwhether the temperature is advanced rapidly during the first part of thecook and slowly during the latter part or advanced slowly during thefirst part and rapidly during the latterpart ofthe cook.

The rate of advance of temperature is entircly a matter of control ofthe relief val-ve on the cover of the digester; hence it is veryimportant for the pressure in the relief linesto be obsolutely constantso that when the cook sets his valve in a certain position he knows hewill get a constant amount of relief.

delicate and requires the constant attention of a'highly skilledoperator. The operator must gauge the condition of the digester byreference to the temperature of the digester as shown by the recordingthermometer which is usually connected at one side of each digester.temperature are, because of the large mass of material involved,transmitted slowly, and the temperature record does not indicateabsolutely the temperature condition throughout the digester. The recordtends to lag behind the actual variations in temperature as they occur.Consequently the most skillful operators find it difficult to maintain auniform and satisfactory operation under the most favorable conditionswhich may be met in digester operation as at present conducted.' It isthe object ofthe present invention to provide a method and apparatuswhereby the difliculties experienced heretofore in the operation ofdigesters and the recovery of acid therefrom are avoided, and wherebystronger acid canbe recovered and utilized more effectively and withoutlosses heretofore incident to the operation of the sulphite process.

Another object of the invention is the-provision of a methodandapparatus whereby the control of the cooking operation is facilitatedand simplified so that a larger number of digesters can be controlled bya single operator to secure more uniform and satisfactory results. IOther objects and advantages of the invention will be apparent as it isbetter understood by reference tothe following specification and theaccompanying drawing, which illustrates diagrammatically the preferredform of the invention.

In carrying out the invention, I employl one or more and preferably aplurality of digesters, and one or more acid recoveryuvessels, towhichthe di gesters may be connected lines connected from theirtops to permitdischarge of the relief gases. I provide, however, a steam main fromwhich steam is supplied to the digesters at constant pressure. This maybe accomplished readily by the introduction offan ordinary pressureregulat ing valve, for example of the diaphragm controlled type, betweenthe source of steam. i. e., a boiler or a reducing turbine, and thesteam main. Thus the pressure may be reduced uniformly from, Aforexample, 150 pounds at the source to 125 pounds per square inch in thesteam main. The steam at constant pressure is then deliveredthroughrecording steam flow meters connected to each `of the digesters. Suchmeters are available needle of the steam flow meter shall follow` thepredetermined chart. 'Ihe amount of steam entering the digester atconstant pressure will thus correspond to the amount predetermined andnecessary to accomplish the intended purpose.

' At the outlet from each of the digesters to the relief line, I providea regulating valve with an auto-matic control, such as a Locke or Masonregulator, which permits the gas to escape from the digesters only whenthe pressure therein reaches a predetermined maximum. Thus, constantpressure caribe maintained in each of the digesters, regardless of the'pressure in the'relief line or lines. The gas escaping from thedigesters passes through the relief line or lines to the acid recoveryvessel, in which it mingles with the acid maintained therein and isdissolved, suitable pressure being maintained in the vessel to ensureanacid of the desired strength. The recovery vessel may be provided witha suitable valve to permit the escape of gas should the pressureriseabove a predetermined maximum.

It is desirable, though not essential, to cool the gas before it entersthe acid recovery vessel. Thus,'where a high free acid is desired,cooling of the relief gases may be effected by introducing a suitablecooling de- Vice in the relief line. The cooling device may be suppliedwith cooling water,-andvtuhe amount of water may be regulated by meansof suitable valves to effect the desired cooling of the gases.Alternatively, the acid inthe recovery vessely may be cooledY by theintroduction of a coil through which water is permitted to flow at thedesired rate to remove the surplus heat, or the acid may be Withdrawnfrom the vessel and circulated through a cooling device and thencereturned to the tank. The purpose of coolingthe acid or the relief gasesis to maintain a relatively low temperature, thus permitting theabsorption and retention of a larger proportion of sulphur dioxide gasat the pressure maintained in the recovery vessel. If dual recoveryvessels arel employed, the relief gases rwill be directed alternatelyinto one or the other of the vessels, depending upon the condition ofthe acid therein.

From the vrecovery vessel or vessels, the

1 acid may be withdrawn from time to time asmay be required anddelivered to the 'several digesters. The strong acid at a predeterminedtemperature can be introduced to the digesters at that temperature, thuspermitting a further control of the operation of the digesters. Coolingof the gases or of the acid involves. a loss of heat. For example,

`the acid may be cooled from an average of 185 F. to a temperature ofapproximately 115 F., involving a loss of approximately 1,000,000' B. t.u. per ton of pulp. The loss represents a cost of only 40e per ton ofpulp. The advantage of operating, however, with strong acid as describedin this application, amounts to from $2.00 to $7.00 per ton, dependingupon the price of wood, which offsets the loss of heat with a very widemargin.

The special advantage of the present invention resides, however, moreparticularly in facilitating the control of the cooking operationthrough the introduction lof steam atconstant pressure and at apredetermined rate. with automatic maintenance of the pressure in thedigester after the pressure.V attains the predetermined maximum. Themethod as described avoids the uncertainty of attempting to control theoperation through manipulation of the relief valve withl the consequentfluctuations both of the pressure in the4 digester and of the rate atwhich the steam is fed thereto. 'With the operation as described, it ispossible to conduct `the cooking at a predetermined and' uniform rateand to control the operation by manipulating a valve in accordance witha visible-index which indicates with certainty the exact conditionswhich are effected when the valve is adjusted.

Referring to the drawing, 5 indicates the digesters of a group, each ofwhich is connected to a relief line 6. AThe relief line is in turnconnected with the bottom of a pressure acid recovery vessel 7. Whilecooling of the relief gases is not essential, I prefer to provide acooler 8 which may be supplied with cooling water through a pipe 9controlled by a valve 10, the water escaping through an outlet pipe 11.`Alternatively other suitable cooling means may be employed, as forexample a coil 12 in the vessel 7 which is supplied 'with cooling waterthrough a pipe 18 controlledV by a valve 14, the water escaping throughra pipe 15. It is to be understood that either or both of the coolingmeans may beemployed or omitted, depending upon the objects to beaccomplish'ed. A pressure release valve 1-6 is provided preferably atthe top of the acid recovery vessel, to avoid excessive pressures in thevessel.

desired point. This valve may be, for example, of the diaphragm-operatedtype, which is commonly employed for the purpose set forth. The.structure of the valve need not be described in detail, as it Will beunderstood by all those skilled in the art.

From the steam main 17 steam lines 20 are connected to each of thedigesters 5 and a steam iow meter 21 is provided in each of the steamlines to indicate the amount of steam at constant pressure which ispermitted to fiow through the lines under the control of valves 22. Thelatter valves are adapted to be manipulated by the operator to permitsteam to iow to the digesters at a predetermined rate. The steam iowmeters are Well known devices which need not be described in detail. Onesuitable type is provided with discs or paper dials adapted to receivean ink line indicating vthe predetermined rate of steam flow. Thesedevices have also a pen on a movable arm, the movement of whichcorresponds to the iiow of steam through the device so that the meterwill record accurately the amount of steam flowing at any given time.The operator may thus adjust the controlling valve so that the amount ofsteam flowing corresponds to the amount indicated by the line on thedial. l

At the top of each digester a manually operable valve 23 is provided,this valve being -normally open so that control of the relief iseffected by an automatically controlled relief valve 24. The control iseffected by a suitable regulator of the Lockey or Mason type, consistingfor example of a diaphragm 25 connected by a pipe 26 to the top of thedigester. The diaphragm is counterbalanced by a Weight 27 to permitpredetermination of pressure to be maintained. The diaphragm is alsoconnected to a Apilot valve 28 which controls the water pressure in a'cylinder 29. A piston 30' in the cylinder is connected to the operatinglever 31 of the valve- 24. Thus as the pressure tends to vary at the topof the digester, the valve 24 is opened or closed and a uniform pressureis maintained.

The gas which escapes from each of the several digesters 5, dependingupon the operation of the valves 24, is delivered through the reliefline 6 to the pressure acid recovery vessel 7 and is there absorbed. Thestrong acid thus produced may be utilized as hereinbefore indicated inthe several digesters. If tWo pressure acid recovery vessels areemployed, the gases may be directed into the vessels alternately.

With the method and apparatus as described, ressure variation in therelief line has no e ect Whatever upon the operation of the digester.Hence the diiiiculties which have been experienced heretofore because ofsuch pressure variations inthe relief line are avoided entirely. Theoperation is simplified, furthermore, by controlling the rate atwhichsteam is supplied to the digesters instead of by attempting to regulatemanually the flow of relief gases from the digesters.

Various changes may made in the details of procedure and in thestructure employed therefor Without departing from the invention orsacrificing any of its advantages.

I claim:

l. The method of cooking sulphite pulp which comprises supplying steamunder manual control to the digester containing the pulp at apredeterminedand regulated rate of iow While maintaining a substantiallyconstant pressure in the digester.

2. The method of cooking sulphite pulp which comprises supplying steamunder manual .control at constant pressure to the digester containingthe pulp at a predetermined and regulated rate of flow While maintaininga substantially constant pressure in the digester.

3. The method of cooking sulphite pulp which comprises supplying steamunder manual control to the digester containing the pulp at apredetermined and regulated rate of flow, and automatically controllingthe delivery of relief gases from the digester.

4. The method of cooking sulphite pulp which comprises supplying steamunder manual control to the digester containing the pulp at apredetermined and regulated rate of flow, and permitting relief gases to.escape from the digester only when the pressure therein attains apredetermined maximum.

5. The method of cooking sulphite pulp Which comprises supplying steamunder manual control at constant pressure to the digester containing thepulp at a predetermined and regulated'rate of iow, and autoj maticallycontrolling the delivery of relief gases from the digester.

6. The method of cooking sulphite pulp which comprises supplying steamunder manual control at constant pressure to the digester. containingthe pulp at a predetermined and regulated rate of flow, and permittingrelief gases to escape from the digester only when the pressure thereinattains a predetermined maximum.

7. The method of cooking sulphite pulp which comprises supplying steamunder manual control to the digester containing the pulp at apredetermined and regulated rate of flow, automatically controlling thedelivery of relief gases from the digester, and dischargin the reliefgases into an acid absorbing me ium.

8. The method of cooking sulphite pulp which comprises supplying steamunder manual control to the digester containing the pulp at apredetermined and regulated rate of flow, permitting -relief gases toescape from the digester when the pressure therein attains apredetermined maximum, maintaining the pressure automatically, anddischarging the relief gases into an acid absorbing medlum.

9. The method of cooking sulphite pulp which comprises supplyingsteamunder manual control at constant pressure tothe digester containing thepulp at a predetermined and regulated rate of flow, automaticallycontrolling the delivery of relief gases from the digester,.anddischarging the relief gases into an acid absorbing medium.I

10. The method of cooking sulphite pulp Which comprises supplying steamunder manual control at constant pressure to the digester containing thepulp at a predetermined and regulated rate of flow, permitting reliefgases to escape from the digester When the pressuretherein attains apredetermined maximum, maintaining this pressure automatically, anddischarging the relief gases into an acid absorbin medlum.

11. The method o cooking sulphite pulp which comprises supplying steamunder manual control tothe digester containing the pulp at apredetermined and regulated rate offlow While maintaininga substantiallyconstant pressure in the digester and Withdrawing and discharging thereliefgases into an acid absorbing medium maintained under pressure.

ing the ulp at predetermined and regulated' flow and automaticallycontrolling -rates o the delivery of relief gases from each of thedigesters. f

14. The method of cooking sulphite pulp which comprises supplying steamunder man- 'ual contro lated rates of ovv, automatically controlling'the delivery of relief gases from each of the digesters, and discharginthe relief ases through a common relief n sorbing` medium.

15. he method of cooking'sulphite pulp which com rises su plying steamunder manual contro toA a p urality of di esters" containing' the pul atpre'determine and regulated rates of ow, the delivery of relief gasesfrom each of the digesters, discharging the relief gasesthrough -acommon relief line tol an acid absorbin A medium in amtalned underfessure, andrcoo 05 mg the acid absorbing me um.

12. The method of cooking sulphite pulpv to a plurality of digesterscons taining thepup at predetermined and reguautomatically controllingltaining the pulp at predetermined and` regulated rates of Howautomatically controlling .the delivery of relief gases from each of thedigesters, discharging the relief ases through a common relief line toan 4aci absorblng medium maintained under pressure,

and cooling the gases before they enter the- 16. The methodof cookingsulphite pulp which comprises supplying steam under manual control to aplurality of digesters conpulp, adigester manually operable means for lsupplying steam at a constant rate of flow to the digester, and meansfor controlling automatically thedischarge of relief gases from thedigester.

. 19. In anapparat-us for cookin sulphite pulp, a digester, manuallyopera le means ,for supplying steam at a constant rate of How to the`digester, means for controlling automatically the discharge of reliefases ,from the digester, and means for absor ing .the relief gases.

20. In an apparatus for cooking sulphite pulp, a` digester, manuallyoperable means or supplying steam at a constant rate of flow to thedigester, means for controlling automatically the discharge of reliefgases -from the digester, means for absorbing the e to` an aci abreliefgases, and means for reducing the temperature of the gas-absorbingmeans.

In testimony whereof I air'ix my si ature.

ROBERT B. OLF.

` los l mi f. ist

